The present invention relates to a process for producing 2,6-dihydroxynaphthalene and highly pure 2,6-diacetoxynaphthalene which are utilized as the monomer of aromatic polyesters having an ability of forming liquid crystalline polymers.
More in detail, the present invention relates to a process for producing 2,6-dihydroxynaphthalene, which comprises oxidizing 2,6-di(2-hydroxy-2-propyl)naphthalene in acetonitrile, 1,4-dioxane or a mixture thereof with hydrogen peroxide in the presence of an inorganic acid or a solid acid at a temperature in the range of room temperature to the boiling point of the solution of 2,6-di(2-hydroxy-2-propyl)naphthalene in acetonitrile or 1,4-dioxane, the acetonitrile, 1,4-dioxane or a mixture thereof being used in an amount of 3 to 30 ml to one gram of the 2,6-di(2-hydroxy-2-propyl)naphthalene, and a process for producing a highly pure 2,6-diacetoxynaphthalene by acetylating the thus obtained 2,6-dihydroxynaphthalene.
In recent years, various engineering plastics have been developed, and as the plastics in this field, aromatic polyesters, particularly those having an ability of forming liquid crystalline polymers have attracted one's attention.
As the starting material for the aromatic polyesters having an ability of forming liquid crystalline polymers, terephthalic acid, hydroquinone, p-hydroxybenzoic acid, etc. may be mentioned, however, in order to improve the physical and/or chemical properties of the aromatic polyesters, not only the benzene compounds but also naphthalene compounds have come to be required as the starting material thereof.
Of the naphthalene compounds, 2,6-dihydroxynaphthalene and 2,6-diacetoxynaphthalene have attracted one's attention as the monomers utilized in the production of the aromatic polyesters from the viewpoints that the physical properties of the liquid crystalline polymers obtained therefrom are excellent and terephthalic acid used as the comonomer thereof is available at a low price.
Although 2,6-diacetoxynaphthalene is obtained by acetylation of 2,6-dihydroxynaphthalene, since 2,6-dihydroxynaphthalene is not produced industrially at present, there is a problem that 2,6-diacetoxynaphthalene is not available inexpensively.
In this connection, as the process for producing 2,6-dihydroxynaphthalene, a classic process has hitherto been known, wherein naphthalene or .beta.-naphthol is sulfonated and the thus sulfonated product is subjected to alkali fusion. Such a process is described in Beilstein's "Handbuch der organischen Chemie". However, according to this process, for instance, as seen from Japanese Patent Publication No. 56-77254 (1981), by-production of the other isomers than 2,6-isomer is inevitable in the sulfonation step and accordingly, the 2,6-isomer cannot be obtained in a high yield. Further, in the alkali fusion step, for instance, as is described in A. P. Kuriakose et al. J. Indian Chem. Soc., 43, 437 (1966), since the yield of 2,6-dihydroxynaphthalene is as low as 50%, the total yield of 2,6-dihydroxynaphthalene in this process wherein naphthalene or .beta.-naphthol is sulfonated and the thus sulfonated product is subjected to alkali fusion is extremely low.
In addition, it is necessary to provide the draining treatment in the step of alkali fusion and accordingly, there is a problem of increased cost. Namely, the process is industrially poor in practicality.
Besides, a process for producing hydroquinone from p-isopropylbenzene, while applying "the cumene process" has hitherto been known (Japanese Patent Publication No. 51-33100 (1976) and the following process for producing 2,6-dihydroxynaphthalene could be considered.
Namely, while applying the process to the production of 2,6-dihydroxynaphthalene, 2,6-diisopropylnaphthalene is oxidized and the thus obtained dihydroperoxide is subjected to acid decomposition to be converted into 2,6-dihydroxynaphthalene [refer to Japanese Patent Applications Laid-Open (KOKAI) Nos. 61-93 156 (1986), 61-100 558 (1986) and 61-191 638 (1986)].
However, in such a process, since the yield of the dihydroperoxide is poor as compared to that of the process for producing hydroquinone and it is difficult to isolate the thus formed dihydroperoxide, the above-mentioned process for producing 2,6-dihydroxynaphthalene cannot be said to be industrially suitable.
On the other hand, a process wherein a compound represented by the formula: ##STR1## wherein Ar represents an aromatic ring, is oxidized in a solvent with hydrogen peroxide in the presence of a strong acid into a phenolic compound represented by the formula, Ar-OH, has been also known (for instance, refer to Japanese Patent Application Laying-Open (KOKAI) No. 52-5718 (1977), Japanese Patent Publication No. 35-7558 (1960), British Patent No. 910,735, TSUNODA and KATO J. Chem. Soc. Japan, 80 (7), 689 (1959) and M. S. Kharasch et al. J. Org. Chem., 15, 748 and 775 (1950)). However, the reaction disclosed in the references are processes for converting (2-hydroxy-2-propyl)benzene, p-di(2-hydroxy-2-propyl)benzene, 1-(2-hydroxy-2-propyl)-4-(2-hydroperoxy-2-propyl)benzene, etc. into phenol or hydroquinone.
Further, U.S. Pat. No. 4,214,105 discloses a process for producing pyrocatechol and hydroquinone wherein phenol is reacted with substantially anhydrous hydrogen peroxide dissolved in a non-aqueous solvent, the reaction being carried out in the presence of a strong acid at a temperature of 20.degree.-150.degree. C. in a reaction mixture containing hydrogen peroxide, solvent, phenol and strong acid.
Also, U.S. Pat. No. 4,503,262 discloses a process for the conversion of 2,6-diisopropylnaphthalene to 2,6-diisopropylnaphthalene hydroperoxide with an oxygen-containing gas in C.sub.5 -C.sub.14 aliphatic hydrocarbons in the presence of a catalytically active heavy metal compound, and discloses the following:
"The 2,6-diisopropylnaphthalene dihydroperoxide of this invention can be converted to the corresponding 2,6-dihydroxynaphthalene which is a known starting monomer for the production of polymers. The convention of the dihydroperoxide to the corresponding dihydroxy compound is well known in the art and involves reaction of the hydroperoxide in an acidic medium. Techniques of this type are disclosed in U.S. Pat. Nos. 3,927,124; 3,884,983; 3,928,469; 3,923,908; and 3,900,423, the disclosures of which are incorporated herein by reference."
However, there is no disclosure concerning the process for converting a compound having naphthalene ring such as di(2-hydroxy-2-propyl)naphthalene, etc. into dihydroxynaphthalenes in the references.
In this connection, it is substantially impossible to obtain 2,6-dihydroxynaphthalene profitably in industrial scale from 2,6-di(2-hydroxy-2-propyl)naphthalene (hereinafter referred to as 2,6-DHPN) by the process disclosed in the references due to the following reason.
Namely in oxidizing 2,6-DHPN in the solvent used in the process disclosed in the references (1) 2,6-DHPN does not dissolve in the solvent described in the references, (2) in spite of the disappearance of 2,6-DHPN from the reaction system, 2,6-dihydroxynaphthalene is scarcely formed, (3) the solvent itself reacts in the system or (4) the reaction rate is very low.
The above-mentioned facts are considered to be due to the remarkable difference of the physical and/or chemical properties between the naphthalene compound (2,6-DHPN) and the benzene compound ((2-hydroxy-2-propyl)benzene, etc.).
As a result of the present inventors' studies on the industrially profitable process for producing 2,6-dihydroxynaphthalene and 2,6-diacetoxynaphthalene, it has been found by the present inventors that (1) 2,6-dihydroxynaphthalene can be profitably produced by using 2,6-DHPN as the starting material, which is available from 2,6-diisopropylnaphthalene which is easily available in an industrial scale and subjecting 2,6-DHPN to oxidation under specified conditions, and (2) 2,6-diacetoxynaphthalene of a high purity can be produced in a high yield by further acetylating the thus obtained 2,6-dihydroxynaphthalene, and based on the findings, the present inventors have attained the present invention.
Namely, the object of the present invention is to provide a process for advantageously and industrially producing 2,6-dihydroxynaphthalene which is useful as the raw material for producing the so-called liquid crystal polymers which has a capacity of forming liquid crystals.